Sounds very similar to what happened to an iMac (late 2013) at work exactly with that update. After installing the update the first time (and using the machine for several hours) the next boot up was black bios type screen stating the OS "could not be found/loaded" and would be in reboot loop. I could select the boot OS drive from bios type menu (after finding a solution to a similar sounding issue from several years ago on the web). Then it would boot up normally but I had to re-apply the update!
So this may not just be "Pro" related?
Also this :"No wonder there are already
$500 discounts on the iMac Pro (all but unprecedented for an Apple Mac only a few months old)." reads to me as jumping to conclusions prematurely.
The discount could simply be as it IS such an expensive computer in the first place rather than just because of teething problems which so far all sound software related. Anything with a higher markup will allow retailers to be a bit more flexible with pricing to shift the units.
Going by the forum here most problems seem to stem from Bootcamp drivers.
I never even saw people post about Bootcamp before with regards to iMac. Never knew it was such a big thing to be honest.